For most Excel functions, involving references ((cell, area), (2d, 3d)), the references are
passed in as arguments, and the exact location remains fixed. However, a select few Excel
functions have the ability to access cells that were not part of any reference passed as an
argument.
Two important functions with this feature are
INDIRECT and
OFFSET
When POI evaluates formulas, each reference argument is capable of evaluating any cell inside
its range. Actually, even cells outside the reference range but on the same sheet can be
evaluated. This allows
OFFSET to be implemented like most other functions - taking only
the arguments, and source cell coordinates.
For the moment this interface only exists to serve the
INDIRECT which can decode
arbitrary text into cell references, and evaluate them..